1. Field of the Invention
The present invention relates mainly to a liquid crystal composition suitable for use in an active matrix (AM) element, and an AM element including the composition.
2. Related Art
On a liquid crystal display element, classification based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insular metal (MIM) and so forth. TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source is a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
These elements include a liquid crystal composition having suitable characteristics. General characteristics of the composition should be improved to obtain an AM element having good general characteristics. Table 1 below summarizes a relationship between the two general characteristics. The general characteristics of the composition will be explained further based on a commercially available AM element. A temperature range of a nematic phase relates to the temperature range in which the element can be used. A desirable range at a higher limit temperature of the nematic phase is 70° C. or more and a desirable range at a lower limit temperature is −20° C. or less. The viscosity of the composition relates to the response time of the element. A short response time is desirable for displaying a moving image. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General characteristics of a liquid crystalcomposition and an AM elementGeneral characteristics of aGeneral characteristics ofNocompositionan AM Element1Temperature range of aUsable temperature range isnematic phase is widewide2Viscosity is small1)Response time is short3Optical anisotropy isContrast ratio is largesuitable4Threshold voltage is lowElectric power consumptionis small and a contrastratio is large5Specific resistance is largeVoltage holding ratio issmall and a contrast ratiois large1)A liquid crystal composition can be injected into a cell in a short time. 
The optical anisotropy of the composition relates to the contrast ratio of the element. A product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the element is designed to be approximately 0.45 micrometers to maximize the contrast ratio of the element. Accordingly, the optical anisotropy of the composition is in the range from 0.08 to 0.12. A low threshold voltage in the composition contributes to a small electric power consumption and a large contrast ratio of the element. Accordingly, a low threshold voltage is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the element. Accordingly, a composition having a large specific resistance in the initial stage is desirable. A composition having a large specific resistance even after it has been used for a long time is desirable.
A composition having a small viscosity, a low threshold voltage and a large specific resistance is especially desired to display a moving image with a large contrast ratio in an AM element. Related compositions are disclosed in the following patent documents. JP H10-204016 A/1998 (U.S. Pat. No. 6,007,740), JP H10-204436 A/1998 (U.S. Pat. No. 5,961,881), JP 2001-003051 A/2001, JP 2001-003053 A/2001, JP 2001-123170 A/2001, JP 2001-288470 A/2001, and WO 2001-046336 A/2001.